In recent image forming apparatuses, a belt fixing system that has a lower heat capacity has attracted attention in response to requests for a shorter warm-up time (a period from when an image forming apparatus is powered on to when fixing by a fixing device becomes ready) and energy saving in the fixing device. Also, in recent years, an electromagnetic induction heating (IH) technique capable of rapid heating and high-efficiency heating has attracted attention as a heating technique adopted in the fixing device. From the viewpoint of energy saving in fixing of color images, a large number of fixing devices utilizing the electromagnetic induction heating technique and the belt fixing system in combination have been commercialized. When the belt fixing system and the electromagnetic induction heating technique are used in combination, a device (coil) for generating magnetic flux for electromagnetic induction heating is often provided on the outside of a heating belt because this arrangement provides advantages such as ease of layout and cooling of the coil and direct heating of the belt (so-called external IH system).
In the above-described electromagnetic induction heating technique, there is known a technique of preventing an excessive temperature rise of the fixing device. More specifically, a fixing roller includes a magnetic shunt alloy layer and a nonmagnetic metal layer, and a coil and the nonmagnetic metal layer oppose each other with the magnetic shunt alloy layer being disposed therebetween. The thickness of the magnetic shunt alloy layer is set to be less than a magnetic-field permeation depth (a surface skin depth) at a temperature higher than or equal to the Curie temperature.
Thus, when the temperature of the magnetic shunt alloy layer is lower than the Curie temperature, a magnetic flux generated by the coil does not reach the nonmagnetic metal layer, and the magnetic shunt alloy layer generates heat. When this heat generation of the magnetic shunt alloy layer increases the temperature of the magnetic shunt alloy layer to be higher than or equal to the Curie temperature, the magnetic flux generated by the coil penetrates the magnetic shunt alloy layer, and reaches the nonmagnetic metal layer, so that an induced current is generated in the nonmagnetic metal layer. The magnetic flux penetrating the magnetic shunt alloy layer and a magnetic flux in a direction opposite the penetrating magnetic flux, which is generated by the induced current in the nonmagnetic metal layer, cancel each other. This suppresses heat generation in the nonmagnetic shunt alloy layer.
However, even when the above techniques of the related art are combined, there remain problems with further reduction of heat capacity of the fixing device. For example, it is conceivable that the heat capacity can be further reduced in the combination by decreasing the thicknesses of the layers. However, if the thicknesses of the layers are simply reduced, the heating efficiency or the magnetic flux reducing effect becomes more likely to decrease.
More specifically, if the thickness of the magnetic shunt alloy layer is reduced, the magnetic flux generated by the coil easily penetrates the magnetic shunt alloy layer. With this, this magnetic flux and the opposite-direction magnetic flux generated by the nonmagnetic metal layer cancel each other, so that the heat generation efficiency of the magnetic shunt alloy layer decreases. In contrast, if the thickness of the nonmagnetic metal layer is reduced, the sectional area where the induced current passes decreases, and the electric resistance of the nonmagnetic metal layer increases. As a result, the opposite magnetic flux is not easily generated.